Boulder, Colo. - The March issue of GEOLOGY covers a wide variety of potentially newsworthy subjects. Topics include: revised age estimate for the Jurassic-Cretaceous boundary; discovery of ejecta from the Sudbury impact event in Canada; new model for predicting landslides; public health concerns from the first regional air assessment downwind from Kilauea volcano after 22 years of continuous eruption; evidence for Early Holocene collapse of the George VI ice shelf; and production of the first sequence of continental summer temperatures during the Antarctic glaciation of the Cenozoic.

Highlights are provided below. Representatives of the media may obtain complimentary copies of articles by contacting Ann Cairns. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Ann Cairns for additional information or other assistance.

GEOLOGY

People have been the main cause of worldwide erosion since early in the first millennium. Many researchers have tried to assess the impact of human activity on soil loss, but most have only guessed at how erosion due to natural forces such as glaciers and rivers compares with that caused by human activity — mainly agriculture and construction. Wilkinson used existing data on sedimentary rock distributions and abundances to calculate rates of natural erosion. If you ask how fast erosion takes place over geologic time — say over the last 500 million years - on average, you get about 60 feet every million years. In those parts of the United States where soil is being eroded by human agricultural activity, however, the rate averages around 1,500 feet per million years, and rates are even higher in other parts of the world. Natural processes operate over areas larger than those affected by agriculture and construction, but even taking that into account, "the bottom line is, we move about 10 times as much sediment as all natural processes put together," Wilkinson said. Because soil formation proceeds at about the same rate as natural erosion, these results mean that humans are stripping soil from the surface of Earth far faster than nature can replace it. This situation is particularly critical because the human population is growing rapidly and almost all potentially arable land is already under the plow.

Bentley et al. report evidence for the collapse of the George VI Ice Shelf in the Antarctic Peninsula, about 9500 years ago. Previous studies by the British Antarctic Survey have shown that some of the ice shelves that collapsed in the late twentieth century also collapsed in the past, but this study is the first to show that a currently healthy ice shelf actually collapsed in the past. In other words, the collapse of ice shelves was more extensive in the past. The collapse of the George VI Ice Shelf occurred after a long period of warmth and at about the same time that there was a shift in ocean currents around the peninsula. The absence of the ice shelf 9500 years ago suggests that natural ocean-atmosphere variability in the Antarctic Peninsula may have been greater in the early part of this interglacial than in recent decades. The study also suggests that the ocean may have played a role in destroying the George VI Ice Shelf. This contrasts to the recent events where ice shelves have been destroyed by rising air temperatures.

Shatsky Rise is a large oceanic plateau (greater in area than California) in the northwest Pacific. Basalt sills were cored recently in Ocean Drilling Program Hole 1213B in the southern part of the rise. Dating of the sills provides an important new age estimate of the boundary between two geological periods, the Jurassic and the Cretaceous; this age is 144.6 ± 0.8 million years before present. The basalt from the sills and from two dredge sites on the rise displays Nd, Pb, and Sr isotope characteristics like those of basalt from the ocean-ridge system. However, the most widely accepted model for the formation of oceanic plateaus, the mantle plume-head model, predicts ocean-island-type isotopic signatures instead. The observed ocean-ridge-type characteristics are compatible with the competing meteorite-impact and plate-separation models, but these models are not supported by other aspects of Shatsky Rise, leaving its origin a mystery.

This manuscript contains groundbreaking results in the field of palaeoclimatology, a subject of universal interest because of its application to understanding present and likely future human-induced climate change. Unravelling past global climate change has seen dramatic revelations in recent years through isotopic analysis. Grimes et al. have used a novel methodology involving a combination of the oxygen isotopes in fossil mammalian tooth enamel and freshwater carbonate organisms to produce the first sequence of continental summer temperatures across a key 3 million year interval that spans the first major Antarctic glaciation of the Cenozoic Era at 33.5 million years ago. The results, in combination with other recently published data, make it almost certain that the majority, if not all, of the isotopic shift recorded in the marine realm at this time is related to a localized cooling and build up of ice on Antarctica and is not related to a significant change in the global climate.

Addison et al. announce the discovery of impact ejecta from the Sudbury, Ontario, Canada, structure, the second largest and third or fourth oldest extraterrestrial Earth impact site. At 1.85 billion years old, these Paleoproterozoic ejecta are three times older than the previous oldest dated ejecta linked to a specific impact (Acraman, Australia, 0.59 billion years old). It is also larger than the well-known Chicxulub, Mexico (0.065 billion years old) impact linked to the extinction of the dinosaurs and many other species. The young Chicxulub impact, particularly its well-preserved worldwide ejecta debris layers, have produced criteria to judge other large ejecta deposits. Foremost is the occurrence of sets of microscopic parallel lamellae in quartz and feldspar grains produced by the intense shock generated at the point of impact. Secondarily, the impact generated a megaplume of vaporized, melted, and crushed crustal rocks, creating molten droplets containing bubbles of gas, and larger accreted balls of dust and rock shards called impact accretionary lapilli. These features, and more, are seen in the Sudbury debris. The debris (ejecta) studied here, landed 650 km west northwest of Sudbury near Thunder Bay, Ontario, Canada, and 875 km west of Sudbury near Hibbing, Minnesota, United States. This huge impact likely deposited debris all around Earth, but it is very difficult to find because so much of the evidence has been destroyed in the recycling of Earth's crust by plate tectonics. Life at the time of the Sudbury impact was confined to the oceans and consisted of unicellular and colonial unicellular organisms. So far, Addison et al. have found no evidence of extinction of this life. However, future studies may link this impact and its ejecta with changes in the classic Gunflint Iron Formation unicellular organisms and their photosynthetic microbial mats, which helped produce Earth's atmospheric oxygen.

Landslides are a major hazard worldwide, killing on average more than 8500 people per year, the majority of whom are buried alive. About 90% of all deaths occur in less developed countries, especially in Central and South America, and South and Southeast Asia. To mitigate against landslides, geologists need to build a better understanding of the processes through which they occur. Petley et al. performed a detailed field and laboratory investigation of catastrophic landslide failures. The aim of their work was to develop a new model for the ways in which a slope goes from being stable to becoming a landslide. They present a new model for this process, and demonstrate the ways in which a sliding surface in the base of the landslide can form. Most landslides undergo small amounts of movement prior to final collapse. Their model allows these movements to be analyzed and permits predictions of future behavior to be made. Importantly, for the dangerous progressive failures, it allows accurate prediction of the time of final collapse and the type of movement that will then occur. Thus, for the first time, it is possible to predict when a catastrophic landslide will happen, which can form the basis of warning systems and better mitigation strategies.

The creeping segment of the San Andreas fault, which extends from San Juan Bautista southeast to Cholame, behaves differently from the remainder of the San Andreas fault. This section of the fault is aseismic, meaning that it moves constantly without major earthquake slip events. Titus et al. have reoccupied 30-year-old alignment arrays on this section of the San Andreas fault and found that the slip rate on the creeping segment is approximately 25 mm/yr. This estimate is consistent with global positioning system (GPS) data for the last 18 months, indicating that the fault creep is relatively constant. This rate is slower than previous estimates and slower than that predicted from plate tectonic models. There are two ways to explain the data, both of which imply higher seismic hazard in central California. First, the "missing slip" might be due to locking along the San Andreas fault, which would imply that significant earthquakes could occur on the creeping section of the fault. Second, the missing slip could be taken up by deformation in the Coast Ranges. This would result in earthquakes on other faults besides the San Andreas, such as occurred in the San Simeon earthquake in December 2003.

Increasing efforts have been made to study the in situ conditions of shallow marine gas hydrate environments. At these sites, cold methane-rich fluids and gases emanate from the seafloor. The methane escapes into the water column and is stored within the sediments in gas hydrates or within carbonates associated to hydrates. Aragonitic clathrites are methane-derived precipitates that are found in close contact with methane hydrates at the summit of southern Hydrate Ridge off the coast of Oregon. By investigating the isotopic composition of the three main components of carbonates which are calcium (d44/40Ca), carbon (d13C), and oxygen (d18O), Teichert et al. have found a new archive that stores the evolution of pore waters surrounding the gas hydrates. With this archive they can monitor in situ the growth of gas hydrates over time. Clathrites also have the potential to tell researchers more about past hydrate deposits because their formation corresponds to times of probably enhanced methane flux and fast gas hydrate growth.

The first regional air assessment was conducted downwind of Kilauea volcano after 22 years of continuous eruption. Volcanic sulfur dioxide gas and fine aerosol pollution were detected at concentrations that justify public health concern for communities along the plume path. A new pattern of plume dispersion depicts an increase of fine aerosol and a decrease of sulfur dioxide with altitude suggesting primary emission of sulfate from Kilauea and oxidation of sulfur dioxide with land-derived oxidants and atmospheric hydrosols.

When a layer of rock is removed from the surface of Earth by erosion, the loss of pressure is sensed, and other rocks from deeper down in Earth will move up to compensate for the removed mass. This is called isostatic rebound. If large glaciers cut deep troughs through mountain ranges isostatic rebound also takes place, but while the incision is localized to valleys, the rebound is spread over a much wider region. Under some circumstances this property can lead to the intriguing result that the mean elevation of the region may be reduced by erosion, but the peaks get higher. This study shows the special past-climate conditions for East Antarctica (frozen mountain tops but erosive wet based glaciers) are favourable for creating this phenomenon. Some of the strongest topographic relief on Earth is observed where the giant outlet glaciers of the East Antarctic ice cap cut through the Transantarctic Mountains, e.g., 5500 m of relief between mountain peaks and the valley bottoms over a lateral distance of just 40 km. Stern et al. calculate that up to 50% of the 4000 m high peak elevations of the Transantarctic Mountains are due to isostatic rebound as a response to the incision of the deep glacial valleys. In contrast, simultaneous erosion of both mountain tops and valley bottoms in temperate climates provides a natural limit to the creation of relief. Estimates from numerous studies in mid latitudes show a maximum 25% of peak elevation can be ascribed to isostatic rebound as a response to incision. Thus, what Stern et al. have identified is a clear role that climate plays in modulating relief and peak height in mountain ranges.

River morphology is increasingly used to reveal information about tectonics in regions where other data are sparse or equivocal. Recent work indicates that river channel width appears to be an important response variable to changes in tectonic, lithologic, and climate forcing. However, at present there is no theory or model that adequately explains this variability. Most models assume width scales with only discharge and therefore cannot address the downstream narrowing of channels observed in a number of settings. Based on the Manning equation and mass conservation, Finnegan et al. derive an expression for channel width that depends not only on discharge, but also on channel slope. The relation explains the dramatic downstream channel narrowing on the Yarlung Tsangpo in eastern Tibet and provides a simple theory for why downstream channel narrowing occurs.

GSA TODAY

The extinction of the dinosaurs in North America
David E. Fastovsky, Department of Geosciences, University of Rhode Island, 9 East Alumni Ave., Kingston, Rhode Island 02881, USA, and Peter M. Sheehan, Department of Geology, Milwaukee Public Museum, 800 West Wells Street, Milwaukee, Wisconsin 53233, USA

Did the dinosaurs really go abruptly extinct 65 million years ago at the Cretaceous-Tertiary ("K-T") time boundary, or was their K-T extinction the end result of a gradual decline over millions of years? The imperfect record available from scarce dinosaur remains has made this question a true enigma and the source of a long-standing and contentious debate. David E. Fastovsky and Peter M. Sheehan conclude, after a careful review of the fossil record from the best-documented dinosaur sites (in North America), that the extinction was indeed geologically instantaneous. The authors argue that the sudden die-off was different in scope from previous fluctuations in dinosaur diversity through the dinosaurs'160 million years on Earth. The authors consider the dinosaurs to have been direct casualties of the K-T impact of an asteroid with Earth and review several potential explanations for the mechanism of the extinction. All of these are concordant with the asteroid impact as the ultimate cause.